Traditionally, the performance of electric power grounding systems has been evaluated by measuring the impedance of the grounding system using the so-called "fall of potential" method. In the fall of potential method, a reference electrode is placed in the ground at a location far removed from the grounding system. A current is applied to the grounding system and the voltage potential between the grounding system and the reference electrode is measured. This voltage potential is referred to in the art as the "ground potential rise" and the method assumes that this ground potential rise or "GPR" is a measurement of the voltage potential between the grounding system and the most remote earth. The GPR simply is divided by the current applied, according to Ohm's Law (V=IR), to determine the resistance of the grounding system. The resistance is assumed to be equal to the impedance, thus providing an estimation of the impedance.
This fall of potential method suffers badly from several deficiencies. For example, very long wires are needed to place the voltage probe as far from the grounding system as possible in an attempt to get clear of the zone of influence or electrical interference in the vicinity of the grounding system. Typically, these wires are 2-3 miles long and some are much longer. However, in order to completely avoid the interference from the grounding system itself, one would have to place the voltage probe at an infinite distance from the grounding system. Obviously, this is not possible. Also, as the length of the wire is increased, the reliability of the signal suffers due to voltage interference induced in the long wire by electric sources, such as power distribution lines, adjacent the wire. Furthermore, where grounding systems are placed in the vicinity of populated areas, the use of long wires is generally impractical owing to problems in securing a needed right-of-way.
By assuming that the measured voltage potential between the grounding system and the probe is equal to the voltage potential between the grounding system and the most remote earth, the fall of potential method introduces a significant error in the estimation of impedance. This is compounded by the simplistic approach used to estimate impedance without considering the reactance component of impedance.
It has been common in the practice of the fall of potential method to use a steady-state, alternating current source for providing the electric current injected into the ground through the grounding system. Unfortunately, such a current source produces currents very much like currents applied to the grounding system by the electric power system, making it difficult to distinguish a useful voltage signal from common noise. Finally, the above-described shortcomings of the fall of potential method render the method generally useless for very large grounding systems having very low impedances, because in extending the probe and the wire to beyond the zone of influence of the grounding system, the probe and the wire typically would end up either in the zone of influence of another grounding system or the interference from power circuits will corrupt the useful signal, or both, and because in this case the useful signal is relatively low, the method is virtually prevented from making an accurate measurement of low impedance values.
Accordingly, it can be seen that a need yet remains for a method and apparatus for measuring ground impedance which does not require long wires, which provides an accurate measurement, and which is effective for evaluating large grounding systems with low impedances. It is to the provision of such a method and apparatus that the present invention is primarily directed.